Demand for heterojunction bipolar transistors HBTs has increased significantly because these transistors are capable of operating at higher speeds and driving more current. These characteristics are important for high-speed, high-frequency communication networks such as those required by cell phones and computers.
HBTs can be used to provide linear voltage and current amplification because small variations of the voltage between the base and emitter terminals, and hence the base current, result in large variations of the current and voltage output at the collector terminal. The transistor also can be used as a switch in digital logic and power switching applications. Such HBTs find application in analog and digital circuits and integrated circuits at all frequencies from audio to radio frequency. Recent advancements in manufacturing transistors with compound semiconductive materials have created a renewed interest in the use of HBTs.
A bipolar junction transistor (BJT) is a three-terminal device that can controllably vary the magnitude of the electrical current that flows between two of the terminals. The three terminals include a base terminal, a collector terminal, and an emitter terminal. The movement of electrical charge carriers produce electrical current flow between the collector and the emitter terminals varies dependent upon variations in the voltage on the base terminal thereby causing the magnitude of the current to vary. Thus, the voltage across the base and emitter terminals controls the current flow through the emitter and collector terminals.
The terminals of a BJT are connected to their respective base, collector and emitter structures formed in a semiconductor substrate. BJTs comprise two p-n junctions placed back-to-back in close proximity to each other, with one of the regions common to both junctions. There is a first junction between the base and the emitter, and a second junction between the emitter and the collector. This forms either a p-n-p or an n-p-n transistor depending upon the characteristics of the semiconductive materials used to form the HBT.
HBTs are BJTs where the emitter-base junction is formed from two different semiconductive materials having similar characteristics. One material used in forming the base-emitter junction preferably is a compound semiconductive material such as silicon and silicon-germanium (SiGe), or silicon-germanium-carbon (SiGeC), or a combination thereof. HBTs using compound semiconductive materials have risen in popularity due to their high-speed and low electrical noise capabilities, coupled with the ability to manufacture them using processing capabilities used in the manufacture of silicon BJTs. HBTs have found use in higher-frequency applications such as cell phones, optical fiber, and other high-frequency applications requiring faster switching transistors, such as satellite communication devices.
Although the use of compound semiconductive materials has proven useful in HBTs, once formed by existing methods, this material is subsequently subjected to multiple thermal cycles, implantations and/or etching processes during the formation steps of the remaining elements of the HBT. Such steps include the deposition and etching of oxide layers, nitride layers and subsequently formed polysilicon layers. Several of these processing steps inherently damage the compound semiconductive material. Etching polysilicon over a compound semiconductive layer, for example, adversely affects the compound semiconductive material because the etchants used do not selectively etch only the polysilicon. Some of the compound semiconductive material is also etched during this processing step, resulting in HBTs that are relatively slower and exhibit relatively poor noise performance compared to other HBTs on the same semiconductor wafer.
Furthermore, to improve the operating speed of a HBT, it is important that the base structure be thin enough to minimize the time it takes electronic charges to move from the emitter to the collector, thereby minimizing the response time of the HBT. It is also important, however, that the base structure have a high concentration of dopant in order to minimize base resistance. Typically, ion implantation techniques are used to form a base layer. However, this technique has the problem of ion channeling, which limits the minimum thickness of the base layer. Another disadvantage of ion implantation is that the compound semiconductive layer is often damaged by the ions during implantation.
Additionally, high-temperature annealing typically is required to drive dopants into the various material layers. This annealing step, however, alters the profile of concentration levels of the dopants within the various layers of semiconductive materials forming the transistor to create undesirable dopant profiles within the various material layers.
Existing methods of manufacturing HBTs still have the problems associated with over-etching, the detrimental effects of ion implantation and annealing, and consistency of manufacturability. Additionally, the processing methods used tend to require relatively long manufacturing cycle times and are expensive due to the requirement of two epitaxy deposition steps.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.